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SIPs

Structural insulated panels have been around for decades but have only recently achieved the interest they deserve. They are large factory made panels including walls floors and roofs and are fixed together on site. A lot of the kit houses in the UK are based on SIPs.

Several factors distinguish them from traditional building methods:

they are factory made and this means:

inclement weather is not a problem during manufacture

factory construction can start while site work such as ground works is carried out

quality control is easier

waste and offcuts are easier to manage and recycle

they lack the traditional roof structure so more space is available in the roof area

they can be made to fine tolerances and this allows for:

accurate fitting of components and particularly airtight barriers. See Airtightness

speed of erection can be down to a few days (Youtube usually has a number of somewhat amusing time-lapse videos of SIPs houses going up (eg. here, here and here)

very high levels of insulation and air tightnessA measure of how leaky a building is to air. In other words, how draughty it might be. There are now standard fan pressure tests to check how air tight a house is and the Building Regulations have minimum standards for all new houses (L1A – Conservation of fuel and power in new dwellings (England)). A much higher degree of air tightness is covered by the Passivhaus standard are achievable. PassivhausSee more on the Passivhaus standard. The PassivHaus Institute has pioneered a standard for low energy buildings. It includes very low energy usage and ways of achieving this. The word is derived from the idea of buildings which are fundamentally low energy and passive solar heated rather than using extra gadgets to heat them. See Passivhaus for the UK branch of the organisation. standard is not a problem in principle though whether a particular manufacturer wants to increase their insulation levels may be a problem. (see an interesting example of problems experienced in achieving thicker insulation in an AECBthe Sustainable Building Association article on low energy houses at the Greenoak developments).

a great degree of flexibility of design is possible because of the panel system. Large areas can be easily spanned and overhangs are not a problem as they can be with masonry. See Structural principles

ecologically sound materials lend themselves to the construction process. The frames are mainly made of softwood and OSBOriented Strand Board which is produced in Scotland from local timber. The spacer members within the panels can be mainly OSB. See Timber as a material and OSB

Drawbacks might include:

the distance that materials have to travel. See Embodied energythe total amount of energy it takes to make a material (or a building). See more on embodied energy

the difficulty of making design changes during construction

lack of thermal massthis is about how much heat something can absorb - so it involves its specific heat capacity and its volume. It can be useful for levelling out the peaks and troughs of temperature within a house. See the page on
thermal mass (though this can be addressed by using heavier internal lining boards). See Thermal mass

Design quality

It needs pointing out that not all SIPs manufacturers are working to the same standard. At one end are companies which simply make and supply basic rectangular panels. While this is fine it misses out on most of the advantages possible with the system. They tend to be timber supply companies who have set up associated facilities to make timber products such as roof trusses and panels, fencing etc. They may not know much about building design, services, Building RegulationsThese are the mass of regulations that cover safety, health, welfare, convenience, energy efficiency etc. in the way buildings are constructed. Not to be confused with Planning consent (which is more to do with whether you can put up the building in the first place). See more on the regulations etc.

At the other end are companies which supply a fully integrated building package with everything carefully designed and detailed. This type of company will be using CADcomputer aided design and CAMComputer aided manufacturing. While they tend to have a set of ‘standard’ designs they will also be able to handle custom designs at not much extra cost per sq m. They should be able to guarantee the overall insulation levels and also the air tightness. See Design it yourself

External wall finishes

It is quite normal to clad the outside of a SIPs house with brick or stone but when you think it through it might be slightly absurd. A timber frame lends itself better to a lightweight cladding rather than heavy masonry. There are many alternatives; timber boarding, shingles, tile or slate hanging, renders, cement fibre sheets etc. A masonry cladding requires concrete strip foundations whereas a lightweight cladding may allow the use of block or screw foundations which can be reusable in the future.

Lightweight cladding is generally considerably thinner than masonry and therefore more of the thickness of the wall can be taken up by insulation. See Rain screensthis is a (usually thin) outer cladding on a wall which prevents rain, snow, etc getting at the structure of the wall behind. see more on rain screens

Internal wall surfaces and services

It is normal to space the internal lining board slightly away from the panel itself. The gap may be around 30mm. This leaves enough room to run pipes and wires without them having to penetrate the structure. See Internal linings

Insulation

On of the best types of insulation for the panels is Warmcell. Not only is it a materials made from paper waste but it has excellent insulation properties and sufficient density to help with decrement delayThis relates to the lag time that insulation itself takes to heat up or cool down. It introduces a delay into the effect of the insulation. This can help level out peaks and troughs of temperature. See the section on Decrement Delay. It does however need to be blown in after the panels are delivered to avoid compaction which might otherwise occur during delivery. Other types of insulation used are phenolic, polyurethane, mineral fibre and polystyrene. See Insulation properties

More information is available on the UKSIPS web site. This is the trade association for SIPs

Building Regulations

Structure

The Building Regulations part AThe Approved documentsApproved documents (England) are detailed publications which come under the English Building Regulations. They are based on tried and tested methods of building and if you follow them you are assured of complying with the Regs. The equivalents for Scotland are the Technical Handbook, for Wales: the Approved documents (Wales), and for N.I. the Technical Booklets, (England) part A, deals with building structures covers the structure of a building. This Approved Document goes into a lot of detail for traditional masonry buildings but almost none for timber frame, steel frame, earth building SIPsStructural Insulated Panels - prefabricated (usually in a factory) timber panels often forming part of an integrated building system and aimed at fast site erection. see more on SIPs etc. For these you will need to consult a structural engineer (while SIPs structures are usually handled by the manufacturer)

Note that compartmentationa term used in the Building Regulations to denote fire resistance between two parts of a building (such as a compartment wall or compartment floor) or between one building and another (for instance a party wall), can be achieved using timber frame construction clad in plasterboard or a similar none combustible material.

Fire safety

With most forms of construction there will be implications concerning fire safety. These are covered in the Building Regulations and you can see examples of how to conform with these in Part BThe Approved documents, (England) part B, deals with fire (Fire Safety)

Site preparation and resistance to contaminants

This section, Part CThe Approved documents, (England) part C, deals with Site preparation and resistance to contaminants (C1) and Resistance to Moisture (C2), covers site remediationthe term applied to the method of dealing with pollutants and contaminants in the ground. The Building Regulations cover this in detail. See info about Approved Document C – Site preparation and resistance to contaminants and moisture along with protection from nasties which might affect the construction and occupants such as damp, rain, radon etc. There is an abridged version of the Approved Document specially for houses.

4 comments to SIPs

Also to answer your question (which I missed!) in general SIPs are completely self-supporting and indeed will support the external skin. They will often be joined by insulated splines meaning that there will be minimal cold bridging. Where extra strength is required, for large open spans, some additional timbers may be required within the depth of the frame to support beams – these should all be designed and supplied by the SIP supplier.

Peters comment about information on SIPS is unhelpful; it’s a pay first to access information I’m afraid. Therefore I recommend browsers to do an internet search for “SIPS”. There is plenty of information freely downloadable.

I assumed SIP’S are self-supporting and don’t need “Post and Beam” methods which increase thermal bridging problems and build costs?